New to Astericks for 20 users need advice

Hi to everyone,I am new to Asterisk. I have a project to install 20 Sip phones in my enterprise. Do I need to install a separate network for Asterisk or I can use our present network which consist of a Windows2012 Domain with Exchange. Do I need a dedicated network Link Or I can connect my Computers behind the phone (please explain how thanks). The least expensive sip phone you can advice me to buy. Which switch and what Speed do my server network card for 20 user without codec or which free codec is the best. Thanks

Probably not, but it depends on the existing traffic on the network and/or the ability of the routers to prioritise traffic.

Exchange has nothing to do with the network. Windows Domains have nothing to do with it. Windows may be providing DHCP, DNS and time services. You can probably safely continue to use it for the first two, but, if at all possible, you should access a suitable set of, four or more, external timeservers directly, is Windows is you only other option.

The network, for Asterisk purposes is the cables, ethernet switches, routers, and the peripheral services mentioned above.